Which RPG provides the most useful resources?
Trick question. I am going to assume they mean which company that released the rpg provides the most resources for their game. I have found that many games, even "dead" ones, will have resources for a given rpg from various fan sites. Sometimes it is like finding a buried treasure when you stumble across a website full of useful material. In fact, often, third party sites will have more resources than the company themselves. This makes it a little hard to describe best company as an rpg might have a lot of resources for it but not by the company.
However onto the rpg company with the best support...
5E Dungeons & Dragons.
-Basic Player Rules and DM's Guide for free. You essentially get the rules for free.
-Sage Advice. Errata and rules clarification. This extends to their presence on twitter and other media sites.
-Unearthed Arcana. This series of unofficial game material does still provide a plethora of new material a group can use.
-Store and Event Locator. This is actually pretty good for finding a local game or store. I use it whenever I am traveling to see what stores I can visit.
-DMs Guild. This is their drivethrurpg site where they sell pdf downloads and allow for third party material to be sold as well. Essentially it is a resource for designers to sell 5E products. However, they also have provided several things for anyone there to use for free such as artwork.
-D&D Beyond. This is their latest endeavor and puts the majority of the rules online for anyone to use. If all goes to plan it will allow for character generation, homebrew material and full stats (monsters, magic items, etc) to be accessed.
-Hidden Resources. The WotC site actually has other resources that are not immediately noticeable from their main page. Such things as a Random Tavern, Die Roller, Adventure Generator, or Name Generator.
There may be other games with more resources (especially in the area of free adventures) but none provide the resources of the scope that WotC does.